New mothers are to be given a significant boost in maternity pay and the right to take six months off work as the Government directs the last Budget before the general election towards wooing women voters.

As a war on tax erupted between the two main parties yesterday, Government sources revealed that a central part of Chancellor Gordon Brown's £4 billion giveaway, to be announced on Wednesday, will be an expected increase in the flat rate of maternity pay from £60 a week to £100 a week. The present 18 weeks maternity leave will be extended to 26 weeks.

The Chancellor is using the Budget to respond to concerns that women voters are turning away from the Government, which they believe has not been as radical as expected and has failed to modernise Britain's struggling public services.

The new maternity package will be bolstered by allowing mothers to claim the new working families tax credit - cash back through the pay packet - from the birth of the baby onwards, effectively targeting most help on those who are least well-off.

Previously it could take up to six months before women could get the extra money.

'We hope this is going to be a very, very good package for women,' said one senior source. 'Our absolute determination has been that the greatest help should be for the poorest women. The aim is that women can have long enough to get to know their new babies, which up until now has been the prerogative of the better-off.'

However, the Chancellor is understood to have rejected demands from unions and other campaigners for a major extension of earnings-related maternity pay.

At the moment women get 90 per cent of their usual earnings paid by the state for the first six weeks, then go on to a much lower flat rate for the rest of their time off.

The maternity pay will be one of a package of measures aimed at making the Budget as 'family friendly' as possible, a move which the Prime Minister has told colleagues is vital to winning the election.

Brown will also announce a large extension of the 10p tax band which will be trumpeted as a 'tax cut for all'. Although he is thought to have rejected an across the board 1p cut in income tax as appearing too profligate, he will say that from April families on average earnings of £25,000 will be £370 a year better off. He will also claim that poorer families on £12,500 a year will be £2,600 a year better off and will have the lowest direct tax burden since 1972.

Yesterday, Michael Portillo, the Shadow Chancellor, upped the war on tax by pledging that he would match any tax cuts announced in the Budget.

Speaking at the Conservatives spring conference in Harrogate, Portillo said the promise came on top of £8 billion of cuts the Tories have already said they can achieve.

He also said that indirect taxes, so-called 'stealth taxes', meant that everyone was now paying the equivalent of 10p extra on the basic rate of income tax.

Labour dismissed the claims, saying that Portillo had a black hole in his spending plans which could only be filled by increasing national borrowing and pushing up interest rates.

The Treasury had initially balked at increasing maternity leave, concerned that it would mainly benefit wealthier high-flyers who are keenest to return to work.

But it has been persuaded by arguments that having a child is the single biggest strain on family incomes - and that if women are allowed to take long enough off to bond with their babies and return only when they are ready, they are less likely to drop out of the labour market. The children's tax credit, which will be given to parents earning up to £40,000, is also likely to be introduced at a higher than expected rate of £10 a week from April.